The objective of this project is the development of a microsensor for glucosewhich can be implanted subcutaneously and can continuously monitor the glucose concentration of the interstitial fluid. This, in turn, can be related to the blood glucose concentration. The sensor sensitivity is defined by the transport characteristics of the outer membrane, a polyurethane composite. Since glucose is polar, it must pass either through pores or defects. Control of membrane permeability is difficult, resulting in a considerable variation in in vitro sensor sensitivity. Therefore the sensor must be calibrated in vivo , a significant inconvenience. Surface analysis of the outer membrane before and after implantation should help make it possible to vary the sensor preparation and materials to obtain a more reproducible permselective outer membrane. ESCA is also being employed to determine suitable sensor coatings.